1. Field of the Invention
The present invention relates to a ceramic multilayer substrate which can constitute an electronic device or an electronic part.
2. Description of the Related Art
In recent years, ceramic multilayer substrates have frequently been used as substrates for circuits which constitute an electronic device or an electronic part. In order to realize high density and high integration of electronic devices and electronic parts, some ceramic multilayer substrates are in the form of a laminate of individual ceramic substrates having different characteristics, for example, combining a low-dielectric constant substrate with a high-dielectric constant substrate.
A typical example of such multilayer ceramic substrates comprises a first substrate comprising a high-dielectric constant ceramic substrate in which a plurality of capacitors are provided and connected to the surface thereof by wiring and on which a thick film circuit has been formed and a second substrate comprising a low-dielectric constant ceramic substrate in which a plurality of capacitors and coils are provided connected to the surface thereof by wiring with a thick film circuit formed on the surface thereof. The two substrates are laminated by a resin or glass such that the opposing conductors are connected. Other electronic parts are mounted on the surface of the first or second ceramic substrates.
However, the conventional multilayer ceramic substrate comprising a plurality of ceramic substrates having different characteristics which are integrated by laminating has the following problems:
(1) When the integrated ceramic substrates have thermal expansion coefficients which greatly differ, the laminated portions of the ceramic substrates are stressed due to temperature drop or heat shock after the ceramic substrates have been laminated by using melted glass, thereby causing cracks. In order to prevent such cracks, it is necessary that the thermal expansion coefficients of the ceramic substrates to be laminated coincide within each temperature range employed during manufacture and use. Attempts have made to add an additive to the substrates for controlling thermal expansion of the ceramic substrates. However, the addition of such an additive can cause deterioration of the other characteristics of the substrates.
(2) Although it is possible to prevent the occurrence of cracks by using a resin having a low Young's modulus to relieve the stress generated in the laminated portion, such a resin cannot be used where high reliability is required because the moisture resistance of the laminated portion deteriorates.